Hi Tom,
Pretty fantastic looking katar! and I am inclined to agree with the Deccan attribution, though I would defer to Jens for the final word on that as he much more familiar with katars.
The weapon that you refer to from Oriental Arms does indeed have a similar shaped blade with reinforced point, however does not of course have the serrated edges. There have been quite a few discussions on this unusual blade feature ( coincidentally on the concurrent thread 'what did I buy? on an unusual polearm).
The sword on Oriental Arms is , as you note, described as Indo-Afghan. This probably derives from other examples with this dynamically flared blade forte that had similiar blades and Afghan form hilts similar to the paluoar. This same blade feature is seen on other Indian sword blades with nagan symbolism and the source of the 'cobra' term. The 'pahari' term is actually used in degree describing 'the mountain people' .
It is interesting to note that it is sometimes difficult to determine where dagger ends and sword begins, as in the case of katars where the blades can sometimes reach 30". The note you mention concerning riveted blades is also interesting, but this was not only used on European blades. In Stone (fig.434 #19) a katar with serpentine blade of 23" riveted to hilt, suggesting this practice applied to unusual blades that are native, as you have noted with yours.
The large discoid form on your blade seems to correspond to certain high quality weapons described in Robert Elgood's "Hindu Arms and Armour" (p.123) as having a chakra at the forte of the blade, rather than the nagan or cobra association in the Oriental Arms example. Although not exactly a disc it seems to align more with that than the flared nagan shape. It is important to note that the 'chakra' or disc, is also known as the circular weapon also termed the quoit and most often associated with Sikh weaponry. In this case the chakra disc alludes to classical Hindu mythology and swords incorporating this device are extremely highly regarded as kingly weapons (p.240). This , along with the very unusual serrated edges, suggest this katar is likely a court weapon.
Rand, excellent reference you have noted concerning the use of the term katar and the misnomer perpetuated by Egerton. This has come up a number of times over years in discussions and much like so many examples of misapplied terms for weapons,the terms have become so entrenched in parlance that efforts to revise them have proven fruitless. Actually I wince as I think of this every time I write on katars, but stick to the well worn term anyway!

Thank you for adding it here for the record.
All best regards,
Jim